The polylactic acid has been known as a resin having excellent biodegradable property. From the standpoint of improving environments, study has now been forwarded in an attempt to use the polylactic acid to substitute for various plastic materials used in various fields of applications. The polylactic acid has now been put into practical use in some fields of applications.
Recently, furthermore, it has been proposed to use the polylactic acid as a dispersion solution for drilling for extracting underground resources (see patent document 1).
To extract underground resources, for example, there has now been widely employed an ore chute drilling method called hydraulic fracturing method. The hydraulic fracturing method consists of pressurizing the fluid filled in the ore chute to form cracks (fractures) in the vicinities of the ore chute to thereby improve permeability in the vicinities of the ore chute (for easy flow of the fluid) in an attempt to increase the effective sectional area through which the resources such as oils and gases flow into the ore chute and, therefore, in order to improve productivity of the ore chute. The fluid is also called fracturing fluid. So far, a viscous fluid like jelly gasoline was used. Recently, however, as the shale gas or the like gas has now been extracted from the shale layer that exists in relatively shallow places and by taking the effects on the environment into consideration, it is becoming a common practice to use an aqueous dispersion solution obtained by dissolving or dispersing a polymer in water. As such a polymer, there has been proposed the polylactic acid.
That is, the polylactic acid is a substance that exhibits hydrolysable capability and biodegradable capability, and, even if it remains under the ground, is decomposed by water or enzyme in the ground and does not adversely affect the environment. Further, the water that is used as a dispersant, too, can be considered to be far from affecting the environment as compared to gasoline or the like.
Further, the ore chute is filled with the aqueous solution in which the polylactic acid is dispersed and the aqueous solution is pressurized so that the polylactic acid permeates into the vicinities of the ore chute. Here, the polylactic acid undergoes the hydrolysis and loses the form of the resin. Therefore, spaces (or cracks) form in the portions where the polylactic acid has permeated accounting for an increase in the space of the ore chute into which the resources can flow.
Here, the polylactic acid is of such a nature that it cannot be easily atomized. For being atomized, the polylactic acid just produced must be subjected to the pulverization and classification (mesh passing) repetitively resulting in an increase in the cost.
Besides, the polylactic acid used for the dispersion solution for drilling must be hydrolyzed. At present, however, the polylactic acid has not almost been studied concerning its pulverizability and hydrolysable capability.
For instance, a patent document 2 discloses a biodegradable resin composition (polylactic acid composition) containing a polylactic acid of which D-isomer content is not more than 2% by mass and a lamellar silicic acid. This polylactic acid composition was developed to improve the heat resistance and the mechanical properties. However, no study has been forwarded yet concerning its pulverizability and hydrolysable capability. In fact, according to the study conducted by the present inventors, the polylactic acid composition of the patent document 2 has a low hydrolysable capability.
Further, a patent document 3 discloses a powder comprising a polylactic acid that has a crystallinity of not less than 30%. The polylactic acid can be mechanically pulverized excellently and can be atomized. However, its hydrolysable capability has not been studied at all.